The present invention relates to a method for PWM (pulse width modulation) frequency correlation, and especially to a method of frequency modulation through PWM for reducing current ripple in a battery.
In a general D. C. (direct current) boost circuit, the PWM signal (referring to the wave shapes of G1-G4 of FIGS. 1 and 2) outputted from two sets of push-pull PWM controller serves to push the MOSFETs (metal-oxide semiconductor field effect transistor) Q1-Q4, and then two transformers T1 and T2 serve to converter the voltage of a battery into a D. C. high voltage.
In the aforesaid structure, the odd PWM signals G1 and G3 are square waves of the same phases which are then inputted to the gates of the MOSFETs Q1, Q3, while the even PWM signals G2 and G4 are square 20 waves of the same phases which are then inputted to the gates of the MOSFETs Q2, Q4. Then, if the MOSFETs Q1-Q4 are triggered by the PWM signals G1-G4 and then are conducted, the working current flowing into the MOSFETs Q1-Q4 are illustrated as the triangular waves of I1-I4 of FIG. 2, while the synthetic current supplied from the battery V BAT is as that shown in I5 of FIG. 2.
From the amplitude (21) of the synthetic current and the D. C. component, it is known that the ripple is very large. Therefore, the inner resistor of the battery V BAT consumes a large amount of power. Even a filtering capacitor C parallel connected to the battery V BAT is burned. This is a problem can not be overcome.
Accordingly, the primary object of the present invention is to provide a method for PWM frequency correlation of the present invention is disclosed. In that, 2xc3x97M PWM signals G1-Gn are supplied to control ends (gates) of the switch elements (such as MOSFET) Q1-Qn of the D. C. boost circuit, where M is a prime number larger than 1, and the PWM signals G1, G3, G5, . . . , Gnxe2x88x921 are synchronous, and have a phase difference of 180 degrees/M, and the PWM signals G2, G4, G6, . . . , Gn are synchronous, and have a phase difference of 180 degrees/M. By this design, a ripple current and EMI interference to the battery as the PWM signal is switched are reduced.
Another object of the present invention is to provide a method for PWM frequency correlation, wherein the ripple component to the battery is reduced effectively and thus, the effect of electromagnetic wave to conductive wires is reduced.
The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawing.